The present invention relates generally to electrical utility meters, and in particular, to a system for the detection of rectified loads coupled to an electricity meter.
Electrical utility service providers, or simply utilities, monitor energy usage by customers through electrical utility meters or electricity meters. Electricity meters track the amount of energy consumed, typically measured in kilowatt-hours, at each customer""s house or facility. The utility uses the consumption information primarily for billing, but also for resource allocation planning and other purposes.
Most utilities generate polyphase electrical power, and typically three phase power. Polyphase electrical power is alternating current electrical power that is supplied on a plurality of power supply lines wherein the voltage waveform as well as the current waveform on each of the power supply lines has a unique phase angle. Generally, only a single phase of the polyphase electrical power is typically provided for single family dwellings while true polyphase electrical power is typically provided to larger facilities such as commercial and industrial facilities.
Polyphase electrical power is provided to customers in a plurality of configurations known as service types. A service type is typically defined by the nominal voltage level and a wiring configuration. A wiring configuration is further defined by the number of wires (three wire or four wire) and the wiring relationship between the phases (wye or delta). A utility meter for polyphase electrical power may be found in allowed U.S. patent application Ser. No. 08/690,973 filed Aug. 1, 1996 by Bond et al. entitled Service Type Recognition in Electrical Utility Meter and specifically incorporated herein by reference. There are, however, many different types of electricity meters including analog, digital, and hybrid analog/digital meters.
A problem with many prior art electricity meters is that when a rectified or DC load is coupled to the electricity meter, the electricity meter cannot accurately measure and/or calculate energy usage. Rectified loads are loads in which at least a portion of the alternating current is rectified. Such loads are typically rare, but nevertheless cause may undesirable metering inaccuracies when they occur. If energy usage is not accurately measured, the utility company cannot equitably bill the user of that energy. Meters that use transformer-based current sensing devices, which are widely used in metering, are particularly vulnerable to inaccuracies due to rectified loads.
Prior art arrangements for detecting a DC component in an AC signal have been developed. U.S. Pat. No. 5,461,306 to Niven shows a DC current detection system for a current transformer that obtains current samples that are spaced apart by 180xc2x0. If the sum of the current samples are substantially zero, then the current waveform is assumed to be symmetric and, as a result, it is determined that no DC current is present. If, however, the sum of the current samples is not zero, then it is determined that a DC current component is present.
One drawback of the technique shown in U.S. Pat. No. 5,461,306 is that it obtains current samples based on the voltage waveform. In particular, the system detects the positive and negative peaks of the voltage waveform and then obtains the current samples that correspond to the detected voltage peaks. The drawback arises from the fact that the voltage peaks and the current peaks do not always coincide, particularly when a capacitive or inductive load is coupled to the system. Although it is still possible to perform the DC current analysis without using the peak current values, the analysis is more prone to error if the peak current values are not used.
What is therefore needed is a system for determining whether a rectified (DC) load is coupled to an electricity meter that is more accurate than the prior art systems. Accordingly, there is a need for a system that more accurately and/or consistently determines peak current values to determine the presence of a DC or half-wave rectified load.
The present invention provides a system, method and apparatus for detecting rectified (DC) loads in an electricity meter by analyzing the current waveform to obtain peak current values. As a result, potential metering inaccuracies due to rectified loads may be uncovered such that corrective action may be taken. Moreover, because the current signal itself is analyzed to generate the peak current values used in the half-wave rectified load determination, the determination is less prone to error than prior art systems.
According to one embodiment, there is provided an electricity meter having a current sensor, an analog to digital (A/D) converter coupled to the current sensor, and a processor coupled to the A/D converter. The current sensor and the A/D converter cooperate to generate a digital current signal having a waveform representative of the current waveform on the power lines connected to the electricity meter. The processor is operable to analyze a digital current waveform signal produced by the current sensor and A/D converter to detect a rectified (DC) load. In particular, the processor perform analysis on the digital current signal to generate a peak positive current value and a peak negative current value, and then detect the presence of a rectified load coupled to the power network based on said peak positive current value and said peak negative current value.
According to another embodiment, there is provided a method for detecting the presence of a rectified load coupled to an electricity meter. The method includes a first step of obtaining a current signal, the current signal representative of current in the electric power network. The method includes another step of performing analysis on the current signal to generate a peak positive current value and a peak negative current value. Then, it is determined whether a rectified load is coupled to the electric power network based on the peak positive current value and the peak negative current value. Finally, the method includes a step of providing an indication of the presence of a rectified load if it is determined that a rectified load is coupled to the electric power network.
The present invention thus provides a method and apparatus for detecting a DC or half-wave rectified load connected to an electricity meter, wherein the method and apparatus determine peak current values using analysis of the current signal itself. As a result, the present invention can determine the presence of a DC component using the more robust peak current values.